I’ll try to keep this short but it is a concern that comes up from time to time while inspecting houses in South Texas. It is not unusual to see water stains or even black moldy-looking areas near or around A/C vents or "registers" as they are called in the trade. I’m often asked what causes that and how can it be fixed. Below is a photo of the problem.

Like most inspectors, I am not a licensed HVAC professional but I have a working knowledge of this problem. As a general rule, the problem is caused by condensation when cold air in the A/C system encounters warm humid air. I know that is an oversimplified explanation.

I also know several first-class HVAC professionals so I put the question to Derek Stewart owner of Aircon Houston. Derek is very knowledgeable and was very willing to let me share his answer with you. So, I am going to quote from his response.

“Ed, Good to hear from you! You are correct in that mildew on the registers is caused by condensation on the grill. The cold air from the AC is mixed with humid air in the room and condensation can happen. Water stains around the register point to two things:

1. The insulation in the register box is old and/or falling off. That insulation can be replaced simply by removing the register, removing the old insulation, and installing new insulation.

2. The insulated box is not sealed well to the surrounding sheet rock. This allows attic air to infiltrate down into the living space. The solution is to remove the register, caulk to seal the area, and reinstall the register.

Most of the time we see the mildew on grills in a bathroom or a room where the owner has shut off airflow (such as in a bedroom that is not used). The real question to ask is why is that room so humid? It could be a faulty vent fan (as in a bathroom) or it could be a system-wide problem with the AC. At times there may be a drain problem that caused undue humid air to be blown into the house. Even after fixing the problem, the mildew will remain on the register. In short, there’s no one answer. But these are the most common.”

Derek Stewart, AirCon Service Co., 281-488-4357, www.airconhouston.com

Everything Derek said regarding this problem makes good sense to me and I want to thank him again for allowing me to share his wisdom. I will add a couple of comments of my own regarding the removal of existing mildew and water stains.

1. A mild solution of bleach water will remove most mildew-type stains pretty easily. I would recommend the use of care and rubber gloves whenever using bleach. A very small drop or drip on just about any fabric will change the color and possibly damage the fabric.

2. After you have addressed the cause and cleaned the area you may need to re-paint to cover the remaining stains. Be sure to use a good primer to keep the stains from bleeding through your final paint job.